As is well known, the core used to form the internal passages, ribs, and heat transfer enhancement means for investment cast air cooled turbine blades or stator vanes is fabricated from a ceramic material. This material inherently is brittle and tends to break if not designed and handled properly. Because of structural integrity considerations, the ribs, particularly at the trailing edge, contain certain constraints which are not necessarily advantages from a cooling aspect. Hence, for example, the flow cavities that serve to cool the trailing edge are typically an array of closely spaced flow channels defined by the ribs and serve to meter cooling flow to obtain optimized cooling effectiveness. Because of these constraints, these flow channels cannot be made sufficiently narrow and shallow to satisfy this requirement. The ceramic cores either become too fragile to handle or cannot survive the casting process. Notwithstanding the above deficiencies, it is also desirable to provide for the cooling effectiveness means by which the cooling flow rate can be adjusted.
The problems attributed to in the above description are exemplified, for example, in U.S. Pat. Nos. 4,515,523 granted to W. E. North, et al on May 7, 1985 and 4,526,512 granted to R. B. Hook on Jul. 2, 1985 which are also incorporated herein for reference.
The U.S. Pat. No. 4,515,523 discloses the use of ribs for structural support of the trailing edge and pin fins and protuberances extending from the ribs for heat transfer enhancement. The U.S. Pat. No. 4,526,512 discloses a spool for a flow control body disposed at the trailing edge of the airfoil of a stator vane for controlling the flow exiting the trailing edge flow channels.
Obviously, the impediment to flow created by the heat transfer enhancing mechanism and the structural ribs to some extent control the flow through the trailing edge. Due to the nature of convective cooling, the slots and the opening between the pins, pedestals and the like are small and the tolerances occasioned in castings of this type produce large variations in these openings and hence the flow at different locations along the trailing edge vary considerably. This, of course, is a problem that needs to be corrected in order to attain maximum life out of the member being cooled as well as conserving cooling air, which impacts engine performance. Typically, in heretofore methods of fabricating the blade, the blade is flow tested in a well known manner, and pursuant to the results of the flow tests, the die for making the ceramic core is modified with the aim of modifying the core and correcting the flow or pressure deficiencies. This is an expensive and time-consuming process, and whenever time is allowed in a given development program, this procedure can be repeated, obviously compounding the expenses and time problems.
We have found that we can obviate the problems outlined in the preceding paragraphs and provide a means and method for controlling the rate of flow of the cooling air in the trailing edge. To this end, we provide restrictions either in proximity to the structured ribs or in the ribs themselves that serve to meter the flow in the trailing edge slots or flow channels. The method of tuning in or tailoring the flow is by incorporating into the ceramic core the openings that will define these restrictions and undersizing these openings. After the blade is cast, the core is leached out. The blade is then flow tested to establish a datum from which the restrictor size required for proper flow and pressure is ascertained. Obtaining the restrictor size becomes very simple and routine merely by enlarging the openings in the core, which have been intentionally undersized, to the desired dimension. Hence, the core is modified to the new dimensions without having to adjust the core dies. The sizing can be done in the cleaning process of the core when the flashing is removed.